It's because you've reached the maximum wattage of the built-in power supply. Because the voltage is so high, the amperage must be reduced to keep within the power supplies rated wattage...
Watts = Amps x Volts
I would also imagine that as the cell voltage rises, the charge amperage might start to fall. Or perhaps the charger is smart enough to know that if it charges 12 Nimh cells in series, the power supply can only deliver 4.1a when they hit their peak voltage.
I'm guessing it's the later situation.
***edit: The max charge wattage for the AC/DC Silent Charger is 60watts. This means at 4.1amps, it can only support a peak voltage of 14.63v. If you had 14 cells, you would only be able to charge at about 3.6amps.
If you want to charge at a higher amperage, you must lower the cell count...